Visible light communication (VLC) is a new technology for short-range optical wireless communications using visible light in optically transparent media. This technology provides access to several hundred THz of unlicensed spectrum, immunity to electromagnetic interference, and noninterference with Radio Frequency (RF) systems. VLC technology also provides additional security by allowing the user to see the communication channel and communication that augments and complements existing services (e.g., illumination, display, indication, decoration, etc.) from visible-light infrastructures. VLC technology has been proposed for use in Intelligent Transport Systems (ITS) for communicating safety and other information between vehicles or between traffic light and vehicles.
LOS (line-of-sight) communication between two VLC transceivers constitutes the majority of the applications in VLC systems. An LOS link is preferred since visible light cannot go through opaque obstacles, such as a wall. However, temporal blocking, such as a walking person, can cause frequent burst frame errors in a VLC system. Also, poor pointing of the VLC device may cause a decrease of signal quality or even link disconnection. VLC systems may also be affected by light dimming (e.g., infrastructure light dimming). When the light is dimmed, the connected link may suffer reduced transmitting time due to pulse width modulation use and/or some data loss due to decrease in signal quality. Since VLC is highly directional, it is difficult to establish and maintain communication links between wireless devices that are mobile. Furthermore, VLC directional behavior makes it difficult to re-establish link that has been lost due to movement or rotation of one of the devices in the link.
To overcome disruptions, at least one prior art system proposes that the access point (AP) provides fast link recovery service in case of the link failure. The AP allocates a dedicated mini-slot in the uplink (UL) for each user equipment (UE), also called a mobile node (MN) or mobile device. Thereafter, the MN transmits signals in the dedicated mini-slot in every frame until the MN disassociates with the AP. However, in this approach, the MN always sends signaling in the dedicated mini-slot for every frame. This may result in a large amount of signaling overhead that wastes system resources. The dedicated mini-slot approach also may consume a lot of battery power at the mobile node.
Another optical communication system, known as Infrared Data Association (IrDA), uses a Serial Infrared Link Access Protocol (IrLAP) to provide a point-to-point connection. An IrDA system has link recovery by a functionality called link reset and uses CRC for error detection. To cope with signal corruption or disruption, IrLAP uses a sequenced information exchange scheme with acknowledgments. If a frame is corrupted by noise, the CRC highlights the error and the frame is discarded. The IrLAP protocol implements an automatic repeat request strategy with options of using stop and wait, go back to N, and selective reject retransmission schemes. This strategy allows the IrLAP layer to provide an error-free, reliable link to the upper layers.
However, the approach used in IrDA does not consider how to support different requirement in battery life of the mobile nodes. Some VLC devices (e.g., infrastructure lights) use AC power and there is no consideration of the battery life. However, for mobile nodes, battery life is an important consideration. However, the IrDA link recovery approach does not consider power management techniques to save battery life. In addition, since light dimming can affect the link conditions, the link recovery needs to consider the dimming factor. However, the IrDA protocol does not consider how to support the light dimming.
Therefore, there is a need in the art for an improved VLC system that is less susceptible to disruptions caused by temporal blocking, dimming, poor pointing, and movement. In particular, there is a need for a VLC system that provides fast link recovery after disruption in both point-to-point connections and point-to-multipoint connections.